Porous microsphere resonators

ABSTRACT

There are several different applications where it is desirable to increase the amount of material that can be introduced to the surface of a microresonator that has whispering gallery modes. The use of a porous surface on the microresonator permits greater amounts of the material to be captured on or near the surface of the microresonator, resulting in an increased optical interaction between the material and the light propagating in the whispering gallery mode(s) of the microresonator.

FIELD OF THE INVENTION

The present invention is directed generally to optical devices, and moreparticularly to optical devices based on microresonators.

BACKGROUND

Dielectric microspheres or planar waveguide ring- or disk-shapedresonators have attracted increasing attention in sensing applications,including biosensing. The size of these types of resonators typicallyranges from approximately 20 μm to a few millimeters for microspheresand from 5 μm to a few tens of microns for ring- or disk-shapedresonators. Such microspheres and ring- or disk-shaped resonators areoften referred to as microresonators. In the most common configurationin microresonator-based biosensors, a microresonator is placed in aclose proximity to an optical waveguide such as optical fiber whosegeometry has been specifically tailored, for example tapered or etchedto a size of 1-5 μm.

The tapering modifications to the waveguide result in there being asubstantial optical field outside the waveguide, and thus light cancouple into the microresonator and excite its eigenmodes, often referredto as whispering gallery modes (WGMs). When microresonators made withlow loss materials and with high surface quality are used, thepropagation loss of light propagating in WGMs is very low, and extremelyhigh quality factors, also known as Q-factors, can be achieved: valuesas high as 109 are achievable. Due to the high Q-factor, the light cancirculate inside the microsphere resonator for a very long time, thusleading to a very large field enhancement in the cavity mode, and a verylong effective light path. This makes such devices useful for non-linearoptical and sensing applications. In sensing applications, the samplesto be sensed are placed on the sphere's surface, where they interactwith the evanescent portion of the WGM available outside themicroresonator. Due to the enhanced field and the increased interactionlength between the light and samples, the microresonator-based opticalsensors feature high sensitivity and/or a low detection limit.

SUMMARY OF THE INVENTION

There exist different applications where it is desirable to increase theamount of material that is to be introduced to the surface of amicroresonator. The use of a porous surface permits greater amounts ofthe material to be captured on or near the surface of themicroresonator, resulting in an increased optical interaction betweenthe material and the light propagating within the microresonator.Accordingly, part of the invention is directed to the inclusion of aporous material at least on the surface of the microresonator.

One such application is where the microresonator is used in a sensor todetect an analyte. Despite the high sensitivity associated withmicroresonator-based sensors, there remains a need to further increasethe sensitivity of such devices, thus lowering the detection limit. Thehigh sensitivity of microsphere resonators lies, in part, in the largeeffective interaction area between the light in WGMs and the samples onthe sphere surface, as discussed in the previous section. The use of aporous surface permits more of the analyte to take part in the opticalinteraction with the light propagating within the microresonator. Oneparticular embodiment of the invention is directed to a microresonatordevice that comprises a light source to produce light and a firstwaveguide coupled to receive the light from the light source. At leastone microresonator is disposed to couple light into the microsphere fromthe first waveguide. The microresonator defines whispering gallery modesand has at least a porous surface region.

Another embodiment of the invention is directed to a method of detectingan analyte. The method includes passing light into a first waveguide andcoupling light from the first waveguide into a microresonator having aporous surface region. The porous coupling region is covered with afluid containing the analyte. Light from the microresonator is monitoredand the presence of the analyte is determined from the monitored light.

Another embodiment of the invention is directed to a microresonator thathas a body operative as a microresonator, defining whispering gallerymodes at at least a first wavelength. At least an outer portion of thebody is porous.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIGS. 1A and 1B schematically illustrate embodiments of amicroresonator-based device;

FIG. 2 schematically illustrates electric field distribution for lightpropagating within a microresonator;

FIG. 3 schematically illustrates electric field distribution for lightpropagating within an embodiment of a microresonator according toprinciples of the present invention;

FIG. 4 schematically illustrates another embodiment of a microresonatoraccording to principles of the present invention;

FIG. 5 presents a graph showing experimentally obtained porous filmthickness for various samples fabricated according to the presentinvention;

FIG. 6 presents a graph showing experimentally obtained values ofrefractive index for various samples fabricated according to the presentinvention; and

FIGS. 7 and 8 present graphs showing experimentally obtained resonanceplots for different embodiments of microresonator fabricated inaccordance with principles of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to optical devices that usemicroresonators, such as microspheres and micro-planar ring- ordisk-shaped resonators for active and/or passive applications. Theinvention is believed to be particularly useful for using suchmicroresonators in sensing applications where the material to be sensedis placed on or near the surface of the microresonator.

An exemplary system 100 that uses a microresonator is schematicallyillustrated in FIG. 1A. This particular system 100 may be used in asensor device. A light source 102 directs light along a waveguide 104 toa detector unit 106. The microresonator 110 is optically coupled to thewaveguide 104. Light 108 from the light source 102 is launched into thewaveguide 104 and propagates towards the detector unit 106. Themicroresonator 110 evanescently couples some of the light 108 out of thewaveguide 104. The out-coupled light 112 propagates within themicroresonator 110 at one of the resonant frequencies of themicroresonator 110.

The light source 102 may be any suitable type of light source. Forincreased efficiency and sensitivity, it is advantageous that the lightsource produces light that is efficiently coupled into the waveguide104, for example a laser such as a laser diode. The light source 104generates light 108 at a wavelength that interacts with the speciesbeing sensed. The light source 104 may be tunable and may or may notgenerate light 108 in a single longitudinal mode.

The light source 102 may direct light into a number of differentwaveguides, of which the waveguide 104 is one. The waveguide 104 may beany suitable type of waveguide and may be, for example, a planar orchannel waveguide formed in or on a substrate, such as a waveguideformed in a silica substrate. The waveguide 104 may also be an opticalfiber.

The detector unit 106 includes a light detector, for example aphotodiode or phototransistor, to detect light. The detector unit mayalso include a wavelength selective device that determines thewavelength of light reaching the light detector. The wavelengthselective device may be, for example, a filter, or a spectrometer. Thewavelength selective device, for example a spectrometer, may be tunableso as to permit the user to actively change the wavelength of lightincident on the light detector.

The microresonator 110 is positioned in contact with, or very close to,the waveguide 104 so that a portion of the light 106 propagating alongthe waveguide 104 is evanescently coupled into the microresonator 110.The waveguide 104 typically has no cladding, or very thin cladding, atthe point where the microresonator 110 couples to the waveguide 104, sothat the microresonator 110 couples directly to the waveguide core.

Another type of microresonator device 150 is schematically illustratedin FIG. 1B. In this device 150, light 158 from the microresonator 110 iscoupled into a second waveguide 154, and propagates to the detector 106.In this configuration, the detector 106 does not detect light from thelight source 102 directly.

Light propagates within the microresonator 110 in so-called “whisperinggallery modes”, an example of which is schematically illustrated in FIG.2. In a whispering gallery mode (WGM) 202, the light propagates aroundthe microresonator 210 from an origin via a number of total internalreflections, until it returns to the origin. In the illustratedembodiment, the WGM 202 includes eight total internal reflections in asingle round trip. It will be appreciated that the light may propagatewithin the microresonator 210 in other WGMs that correspond to differentnumbers of total internal reflections.

Furthermore, the WGM of the microresonator 210 is a high Q mode forlight whose wavelength is equal to an integral fraction of the roundtrip length of the whispering gallery mode. Stated another way, thewhispering gallery mode only demonstrates a high Q where the light is ofsuch a wavelength that it constructively interferes after one roundtrip. This resonant condition can be stated mathematically as:λ _(m) =L/m   (1)

where λ_(m) is the wavelength of the mth mode, L is the optical lengthof one round trip of the WGM, and m is an integer, referred to as themode number. The microresonator efficiently couples in light from thewaveguide 104 that satisfies the resonant condition (1).

The amplitude of the WGM, where the WGM is incident with the side of themicroresonator 210, peaks at the interior surface of the microresonator210. The amplitude of the WGM decays exponentially outside themicroresonator 210, with an exponential decay factor, d, given by d≈λ/nwhere λ is the wavelength of the light and n is the refractive index ofthe medium outside the microresonator 210. The field amplitude isschematically illustrated in FIG. 2 for the WGM 202 along thecross-section line AA′.

Samples of analyte that are to be sensed are introduced to the surfaceof the microresonator. In one method, at least the porous region of themicroresonator's surface, or optionally the entire microresonator, isexposed to a fluid containing the analyte. For example, the fluid may bea liquid containing the analyte in solution or suspension, or may be agaseous mixture containing the analyte. The light-matter interactionbetween the WGM and the analyte on the surface of the microresonator isachieved through evanescent coupling.

As the light-matter interaction is proportional to the strength of thelocal electric field of the WGM, one way to enhance sensitivity is tohave samples experience the maximum electric field and hence achieve themaximum interaction with the WGM. In addition, the refractive indexchange between the interior and exterior of the microresonator dependson the surface adsorption area and the sample molecular weight. A largerinteraction area is desirable, particularly for the detection of smallmolecular weight molecules that form monolayers or only very thinlayers.

According to the present invention, the microresonator is formed usingporous material at least at the surface of the microresonator. Forexample, the microresonator may be formed wholly out of porous materialor may be formed from a central core of non-porous material, with acoating of porous material surrounding the non-porous core. In eithercase, the size of pore of the porous material should advantageously beless than the wavelength of the probe light so as to avoid anysignificant impact on the Q-factor.

One particular embodiment of a layered type of porous microresonator 300is schematically illustrated in FIG. 3, having a porous layer 302 over acore 304. The core 304 may be formed of a non-porous material or may bea hollow core. The electric field of the WGM is pulled toward the porouslayer 302, compared to the case where the porous layer 302 is replacedwith air (or solution such as water). Therefore, the electric field hasa larger portion in the porous layer 302, as illustrated in FIG. 3,where sensing occurs, enabling the analytes to be exposed to higherlevels of electric field, thus increasing the interaction between thelight and the analytes. Furthermore, the use of the porous layer 302enables the analyte to be denser in the region of the WGM 306, furtherincreasing the light-analyte interaction. The density of the analytedepends, at least in part, on the surface area of the microresonator: anincreased surface area results in more sites to which the analyte maybecome attached, since the analyte tends to form a monolayer over themicroresonator. Use of a porous surface results in an increase in thesurface area. In an example, the surface area of non-porous particles isin the region of around 1 m² per gram of material (1 m²/g). The surfacearea of a porous material may be in the range of 1000 m²/g or more.Where the porous material is a layer of porous material about a core,the thickness of the porous layer may be less than λ, where λ is thewavelength of light being used to excite the WGMs of the microresonator.The thickness of the porous layer may also be less than λ/10.

A variation of the layered microresonator 400 design is schematicallyillustrated in FIG. 4. In this embodiment, the thickness of the porouslayer 402 is not constant all the way around the core 404. One of theadvantages of this embodiment is that, when the portion 406 of theporous layer 402 that is thinnest is placed close to a waveguide 408,scattering losses that occur when coupling light between the WGMs 410and the waveguide 408 are reduced. Light coupled into the WGMs 410 isthen adiabatically transferred to the porous layer 402 for sensing.

The porous surface of the microresonator may be modified to attract adesired analyte. Once the desired analyte has been attracted to theporous surface, or close to the porous surface, the spectral propertiesof the microresonator may be changed. For example, the surface of themicroresonator may be treated to attract a protein, a DNA molecule, avirus, or a bacterium. In an example of a microresonator prepared toattract a protein, the surface of the microresonator may be initiallytreated with either an antibody or an antigen, so that the other of theantibody and antigen is attracted to the microresonator when placed intoan analyte solution. The antigen may be expressed on the cell wall of abacterium, in which case the bacterium may be attracted to themicroresonator.

In an example of a microresonator prepared to attract a DNA molecule,the microresonator may initially be treated by having a specific DNAstrand immobilized on its porous surface. The DNA strand immobilized onthe surface of the microresonator is highly selective and only combineswith its complementary strand (cDNA). A viral biosensor may have acomplementary DNA segment attached to its porous surface. The segment iscomplementary to a portion of a, typically larger, viral DNA molecule.The cDNA segment is preferably sufficiently long that it binds stronglyto the viral DNA molecule. The presence of the viral DNA molecule canthen be determined optically. The use of DNA with microresonators isdescribed further in S. Chan et al., “Nanoscale silicon microcavitiesfor biosensing”, Materials Science and Engineering C 15, pp. 277-282(2001), incorporated herein by reference.

Any suitable type of material may be used for the microresonator. In anapproach in which the microresonator includes a porous layer over acore, the core may be any suitable type of glass, for example: silicaglass; modified silicate glass, such as alkali silicate glass; heavymetal oxide glass; halide glass, such as fluoride glass; oxyhalideglass; chalcogenide glass; and phosphate glass. In addition, the coremay also be hollow, and need not be completely solid. Therefore, thecore may be a hollow body, for example a hollow sphere.

The layer of porous material may be formed from any suitable porousmaterial that is transparent at the wavelength being used for analysis.For example, the porous layer may be made from a sol-gel coating, asurfactant-templated material, a microparticulate coating, a layer ofliquid crystal or the like.

The entire microresonator may also be made from a porous material, forexample the porous glass Vycor™ available from Corning Inc., NY.

In addition, the porous layer may be doped with an optically activematerial. For example, the material of the porous layer may be dopedwith an optically excitable species, such as a rare earth ion, thatproduces optical gain. Thus, the light propagating within themicroresonator may be used to pump the optically excitable species so asto obtain gain and/or laser oscillation at a wavelength amplifiable bythe optically excitable species. The microresonator may oscillate at thewavelength amplifiable by the optically excitable species. Thus, themicroresonator may be used to sense an analyte using a wavelength oflight different from that coupled into the microresonator from thewaveguide.

It will be appreciated that different types of optically active materialmay be introduced to the surface of the microresonator to interact withthe light propagating within the WGM(s) of the microresonator. Theinteraction may, for example, include the absorption of light, theemission of light, or some other type of interaction. For example, anorganic dye may be introduced to the surface of the microresonator,where the light propagating within the microresonator causes the dye tofluoresce. This may even lead to oscillation on a WGM at a wavelength oflight amplified by the material introduced to the microresonatorsurface.

EXAMPLE 1 Porous Layer

A sol-gel approach is used in which pore forming agents such assurfactants are included for forming a porous layer over a core. Thispermits control of the pore sizes that can provide some sizediscrimination, for example, unimodal pores with a diameter in the rangeof about 2 to about 50 nm diameter, with diameter control <10%. WhenStober spheres are used, the diameter range is from about 2 nm to 1000nm or more.

A simple system involves combining tetraethoxysilane, nonionicsurfactant, acid, water, and ethanol to form a coating solution. Thissolution can be applied to the core spheres by dipping or spraying. Thesurfactant can be removed if desired by liquid extraction or heating.The walls of the pores can be functionalized via silane coupling agents.

EXAMPLE 2 Porous Layer

Surfactant-templated silica films were dip-coated onto microsphereresonators and onto silicon substrates. The resonators and the siliconsubstrates were calcined at 400° C. for 15 minutes to remove thesurfactant and leave nanometer diameter unimodal pores. Porosities ofthe films range from ˜25 to ˜50 volume percent, and refractive indicesrange from 1.21-1.34.

In this example, there are, by design, two sets of four samples (eightconditions). One set has film thicknesses that range from 40-55 nm; thesecond set has film thicknesses that range from 230-280 nm. Silicaconcentration in the coating solution is the key factor used to controlthickness. Each set of four samples has two samples with small (˜3 nm)and two samples with large (˜10 nm) pore diameters. Surfactant formulaweight is used to control pore diameter. Note that the pore diameters inboth sets are much smaller than the wavelength of the probe light (>400nm). It is anticipated that these porous coatings help to increase thesensitivity of resonator biosensors through increased surface loading ofanalytes. The coatings presented in this example are silica-based,although, in principle can be metal oxide-, metal sulfide-, metal-, orpolymer-based.

A three-factor, two-level full-factorial designed experiment (eightconditions) was employed to examine the effect of the factors listed inTable I on coating thickness, coating porosity, shrinkage upon heating,refractive index, and pore diameter. Eight resonators (one with eachcondition) and 16 silicon substrates (two each with each condition) werecoated with the surfactant-templated silica films. Each of themicrospheres and one of the coated silicon substrates for each conditionwere calcined (see below for details). The as-made and calcined films onsilicon substrates were characterized extensively by ellipsometry. It isassumed that the calcined films on the silicon substrates are areasonable approximation of the calcined films on the resonators. TABLEI DOE Factors and Levels Factors − + Surfactant type (FW g/mol) CTAB(365) P123 (5150) Surfactant/Silica ratio (V/V) 1.8 3.5 Relative SilicaConcentration¹  0.19 1  ¹Based on ˜0.62 M acid-adjusted-TEOS plus ethanol concentration forthe + condition

The first step in the preparation of coated resonators (and siliconsubstrates), was to synthesize a 2.16 M tetraethoxysilane-basedsolution. From this solution the eight coating solutions were preparedby adding the appropriate amounts of surfactant and ethanol solvent, asshown in Table II.

A 2.16 M silica sol stock solution was prepared by combination in a 9 Lflask of 892 mL absolute ethanol (Aaper Alcohol and Chemical Company,Shelbyville, Ky.), 892 mL tetraethoxysilane (Alfa Aesar, Ward Hill,Mass.), 71.97 mL deionized water (18 MΩ), and 0.0210 mL concentrated HCL(JT Baker, Phillipsburg, N.J.; 29% aqueous). The solution was stirredwith an overhead air-driven stirrer and heated on a programmable hotplate (with temperature probe) for 90 minutes at 60° C. After thesolution cooled it was stored at 0° C. in polypropylene bottles.

To prepare the acid-adjusted TEOS solution referred to in Table II,34.48 mL of the 2.16 M TEOS stock solution from the above paragraph,4.14 mL 0.07 N HCl, and 1.38 mL deionized water (18 MΩ) were combined ina 60 mL polypropylene bottle.

The formulations presented in Table II were used to prepare the 8coating solutions. The prescribed amount of acid-adjusted TEOS solutionwas transferred to a 60 mL polypropylene bottle with a micropipette. Thesurfactant and ethanol were added to the bottle and the contents wereshaken vigorously until a clear homogeneous solution formed. Surfactantswere Pluronic P123 (EO₂₀PO₇₀EO₂₀) (BASF, Mount Olive, N.J.) andcetyltrimethylammonium bromide (CH₃C₁₅H₃₀N(CH₃)₃Br or CTAB; Aldrich,Milwaukee, Wis.). TABLE II DOE Run Order and Formulations Acid-adjustedSurfactant Ethanol Sample Notation StdOrder RunOrder Surfactant TEOSsoln (mL) (g) (mL) 1 −++ 7 1 CTAB 6.67 1.18 13.33 2 −−+ 5 2 CTAB 6.670.59 13.33 3 +−− 2 3 P123 1.25 0.12 18.75 4 −−− 1 4 CTAB 1.25 0.11 18.755 +++ 8 5 P123 6.67 1.23 13.33 6 −+− 3 6 CTAB 1.25 0.22 18.75 7 +−+ 6 7P123 6.67 0.61 13.33 8 ++− 4 8 P123 1.25 0.23 18.75Note:the “notation” column refers to the factors in Table I in the order theyappear, i.e. surfactant type, surfactant/silica ratio, reactantconcentration.

Each coating solution listed in Table II was transferred to a 20 mLscintillation vial. Silicon wafers <100>cut, p-type, B-doped (3″ fromSilicon Sense, Nashua, N.H.) were cut into ˜1 cm×2 cm sections. Thewafers were cleaned by sonicating in a LIQUINOX/deionized water solutionfor 2 minutes. The substrates were then rinsed with deionized water for2 minutes and rinsed with ethanol prior to coating.

Each vial was placed successively in a dip-coating chamber. Two Siwafers and one microsphere were coated in turn with each solution. Thewafers and sphere were held vertical. The immersion and withdrawalspeeds into and out of the coating solution were 0.5 cm/s. The coatingswere allowed to dry in air.

After three days, one wafer of each sample type and each coatedresonator was placed into a 400° C. furnace and calcined for 15 minutes.The time to wait before calcining the samples need not be as long asthis, and may be as short as a few hours.

After the wafer samples had cooled, ellipsometry was performed on thecalcined and as-made films. Data (psi and delta) were measured on thefilms on silicon wafers at 50° and 70° with a Gaertner L116Aellipsometer (Skokie, Ill.) that employs 632.8 nm light from amonochromatic HeNe laser. At least three spots were measured for eachsample. Reported values are averages. Values for the thickness, t, andthe refractive index, n, of the single layer films were determined bythe software provided with the ellipsometer. Parameters for thesubstrate were index Ns=3.850; absorption coefficient Ks=−0.020;wavelength 632.8 nm. Results are reported in Tables III and IV and inFIGS. 5 and 6. TABLE III Thicknesses and Refractive Indices of As-made(Uncalcined) Samples on Silicon Wafers Percent Percent error ErrorSample Notation AVE t (Å) SD t (Å) (SD/AVE) AVE n SD n (SD/AVE) 1 −++4256 265 6.23 1.479 0.001 0.08 2 −−+ 5826 173 2.99 1.486 0.009 0.60 3+−−  675 7 0.99 1.456 0.001 0.03 4 −−−  614 11 1.74 1.449 0.003 0.20 5+++ 3628 244 6.71 1.456 0.028 1.93 6 −+− N/A N/A N/A N/A N/A N/A 7 +−+4348 143 3.29 1.463 0.001 0.03 8 ++− 1037 30 2.90 1.452 0.020 1.38N/A - film was too hazy to get data.

TABLE IV Thicknesses and Refractive Indices of Calcined Samples onSilicon Wafers % error AVE t SD t % error (SD/ porosity Shrinkage SampleNotation (Å) (Å) (SD/AVE) AVE n SD n AVE) (%) (%) 1 −++ 2348 13 0.61.318 0.002 0.13 28 45 2 −−+ 2788 52 1.9 1.232 0.008 0.66 46 52 3 +−−383 13 3.3 1.338 0.008 0.60 24 43 4 −−− 453 12 2.6 1.276 0.001 0.09 3726 5 +++ 2628 146 5.6 1.320 0.020 1.51 28 28 6 −+− 533 18 3.4 1.2180.014 1.16 49 7 +−+ 2815 15 0.5 1.317 0.001 0.09 28 35 8 ++− 415 15 3.51.314 0.007 0.53 29 60Porosity was calculated from the Lorentz-Lorenz relationship:P=100*(1−[((n _(silica) ²1)/(n _(silica) ²+2))/((n _(sample) ²−1)/(n_(sample) ²+2))])The refractive index of silica was assumed to be 1.460. Shrinkage wascalculated form the following equation:S=100*[1−t _(calcined) /t _(as-made)]

FIG. 5 shows a graph with film thickness values, t, for as-made andcalcined samples as measured by ellipsometry. FIG. 6 shows a graph ofrefractive index vales, n, for as-made and calcined samples on siliconwafers, as measured by ellipsometry.

For the data on calcined samples, a main effects plot, interactionsplot, cube plot, and effects analysis (Pareto) were produced for theoutputs: thickness, refractive index, shrinkage, and porosity. Theresults are summarized in Table V. TABLE V Summary of Main EffectsOutput is max Output Important x's when factor is: Thickness SilicaGreater concentration Refractive None (surf type) (smaller) indexShrinkage Surf type/conc Large surf/low conc interaction and smallsurf/high conc Porosity None (surf type) (smaller)Note:an alpha value of 0.1 was used to define significance; effect ofsurfactant type on index was examined with t-test; p = 0.079. Note thatthe shrinkage for sample 6# could not be calculated owing to the inability to measure refractiveindex of the as-made film. Parenthesis indicates not statisticallysignificant in at alpha = 0.1 in the Effects Analysis.

There is a strong correlation between coating thickness and silicaconcentration—as expected. There is no statistically significantcorrelation of coating porosity or refractive index with any of the x's.There is correlation between shrinkage and surfactanttype/concentration. Specifically, the largest values of shrinkage tendto occur with large surfactants and low reactant concentrations or smallsurfactants and high reactant concentrations.

EXAMPLE 3 Experimental Results Using Porous Microresonators

Experiments were carried out to determine the effective Q-factors ofmicroresonators fabricated according to the procedure outlined inExample 2. The experimental arrangement was similar to that shown inFIG. 1A. In each experiment, a fiber 108 was placed in contact with themicroresonator 110 and both were immersed in water. A tunable diodelaser (Velocity 6304 from New Focus) was used as light source 102 withthe wavelength centered at around 630 nm. The wavelength of the lightemitted by the laser was tuned by varying the voltage that drives apiezo-actuator inside the laser. When the wavelength was on-resonancewith one or more of the whispering gallery modes 112 of themicroresonator, the amount of laser power coupled into themicroresonator was increased, leading to a power drop at the detector106.

FIG. 7 shows the detected signal at the detector as a function of lasertuning, where the microresonator was formed according to sample number6, in Table II. The Q-factor of the microresonator was estimated fromthese results to be approximately 1.4×10⁶. FIG. 8 shows the detectedsignal at the detector as a function of laser tuning, where themicroresonator was formed according to sample number 7, in Table II. TheQ-factor of the microresonator was estimated from these results to beapproximately 2.8×10⁶.

These two results confirm that high Q-factors can still be achieved inmicroresonators that have porous outer layers.

As noted above, the present invention is applicable to microresonators,and is believed to be particularly useful where microresonators are usedin sensing applications. The present invention should not be consideredlimited to the particular examples described above, but rather should beunderstood to cover all aspects of the invention as fairly set out inthe attached claims. Various modifications, equivalent processes, aswell as numerous structures to which the present invention may beapplicable will be readily apparent to those of skill in the art towhich the present invention is directed upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

1. A microresonator system, comprising: a light source to produce light;a first waveguide coupled to receive the light from the light source;and at least one microresonator disposed to couple light into themicrosphere from the first waveguide, the microresonator definingwhispering gallery modes and having at least a porous surface region. 2.A system as recited in claim 1, wherein the whispering gallery modes areexcitable by the light coupled into the microresonator from the lightsource via the waveguide and the whispering gallery modes are opticallycoupled to the porous surface region.
 3. A system as recited in claim 1,wherein the microresonator is formed entirely from porous material.
 4. Asystem as recited in claim 1, wherein the microresonator includes aporous outer layer disposed over a core.
 5. A system as recited in claim4, wherein the core is a non-porous core.
 6. A system as recited inclaim 4, wherein the core comprises a hollow body.
 7. A system asrecited in claim 1, further comprising a detector optically coupled toreceive light from the microresonator.
 8. A system as recited in claim7, wherein the detector is optically coupled to receive light from themicroresonator via the first waveguide.
 9. A system as recited in claim7, further comprising a second waveguide coupling light between themicroresonator and the detector.
 10. A system as recited in claim 1,wherein the porous surface region of the microresonator is modified toattract an analyte.
 11. A system as recited in claim 10, wherein theporous surface region of the microresonator is provided with one of anantigen and an associated antibody.
 12. A system as recited in claim 1,further comprising an analyte in a liquid medium, the porous surfaceregion being exposed to the liquid medium.
 13. A system as recited inclaim 1, further comprising an analyte in a gaseous medium, the poroussurface region being exposed to the gaseous medium.
 14. A system asrecited in claim 1, wherein the microresonator comprises asurfactant-templated coating on the porous region.
 15. A system asrecited in claim 1, wherein the porous surface region has a thicknessless than the wavelength of the light provided by the light source. 16.A system as recited in claim 15, wherein the porous surface region has athickness less than one tenth of the wavelength of the light provided bythe light source.
 17. A system as recited in claim 1, further comprisingan optically active material on the porous surface region.
 18. A methodof detecting an analyte, comprising: passing light into a firstwaveguide; coupling light from the first waveguide into a microresonatorhaving a porous surface region; exposing the porous coupling region to afluid containing the analyte; monitoring light from the microresonator;and determining, from the monitored light, presence of the analyte. 19.The method as recited in claim 18, further comprising scanning thewavelength of the light coupled into the microresonator.
 20. The methodas recited in claim 18, wherein the fluid is a gaseous mixturecontaining the analyte.
 21. The method as recited in claim 18, whereinthe fluid is a liquid containing the analyte.
 22. The method as recitedin claim 18, wherein the analyte is a protein.
 23. The method as recitedin claim 18, wherein the analyte is a bacterium.
 24. The method asrecited in claim 18, wherein the analyte is a virus.
 25. Amicroresonator, comprising: a body operative as a microresonator,defining whispering gallery modes at at least a first wavelength, atleast a surface portion of the body being porous.
 26. A microresonatoras recited in claim 25, wherein the body is substantially spherical. 27.A microresonator as recited in claim 25, wherein the body issubstantially planar.
 28. A microresonator as recited in claim 25,wherein the body is formed entirely from porous material.
 29. Amicroresonator as recited in claim 25, wherein the body comprises a coreand a porous outer layer surrounding at least a portion of the core, theporous outer layer forming the porous surface portion.
 30. Amicroresonator as recited in claim 29, wherein a geometrical center ofthe core is coincident with a geometrical center of the porous outerlayer.
 31. A microresonator as recited in claim 29, wherein ageometrical center of the core is not coincident with a geometricalcenter of the porous outer layer.
 32. A microresonator as recited inclaim 29, wherein the thickness of the porous outer layer over the coreis non-uniform.
 33. A microresonator as recited in claim 25, wherein theporous surface portion of the body is formed from a porous silicamaterial.
 34. A microresonator as recited in claim 33, wherein theporous silica material is a calcined porous silica material.
 35. Amicroresonator as recited in claim 25, wherein the body has a corecomprising silica.
 36. A microresonator as recited in claim 25, whereinthe porous surface portion of the body has a porosity in the range from10% to 90%.
 37. A microresonator as recited in claim 25, wherein themicroresonator comprises a surfactant-templated coating on the poroussurface portion.
 38. A microresonator as recited in claim 25, furthercomprising an optically active material on the porous surface portion.